Introduction - Graphene

Graphene is a hot research and industrial topic as it demonstrates innovative mechanical, electrical and optical properties. With a single layer of sp2-bonded carbon atoms arranged in a honeycomb crystal lattice, the nanomaterial is the fundamental building block for all types of graphitic materials.

Keysight Technologies is involved in the development and introduction of advanced atomic force microscopy (AFM) techniques for performing nanoscale analysis of the properties of graphene. There are several research papers published by researchers in many different fields based on results obtained through the use of Keysight AFM instrumentation and techniques. The Keysight 5600LS is the most widely used AFM instrument in the world for performing graphene research. This article discusses the technological advantages of AFM in detail.

High-Resolution AFM Imaging of Graphene and its Derivatives

High-resolution imaging of surface morphology is one of the key benefits of AFM. The ability of the Keysight 5600LS system to capture data at a single-molecule level has been very well demonstrated. Figure 1 illustrates high-resolution AFM imaging of graphene and related nanomaterials.

In the last two decades, AFM has been advanced from an instrument capable of capturing only the morphology of a sample to one that can analyze additional materials properties. One such advancement is a single-pass KFM and simultaneous dC/dZ imaging technique developed by the introduction of a carefully designed triple lock-in amplifier (LIA) configuration into the AFM electronics. It is the combined use of a probe flexuralresonance frequency ωmech in the first LIA with a feedback loop for surface profiling, a much lower frequency ωelec in the second LIA with a second feedback control to negate the electrostatic interactions between the tip and sample for quantitatively measuring sample surface potentials, and the second harmonic 2ωelec in the third LIA to observe the oscillation of the tip at this frequency attributable to the electrostatic interactions for dC/dZ signal extraction.

Unlike the conventional KFM that operates through a two-pass approach or lift mode, Keysight KFM operates in the intermittent contact regime, thus always bringing the tip into the vicinity of the sample surface, which, in turn, considerably improves both spatial resolution and detection sensitivity.

Applications of KFM

Graphene’s outstanding electronic properties are the subject of intense research. Graphene sheets need to be supported on an insulating substrate in order to make them useful for nanoelectronic devices such as ultrathin transistors and sensors. Hence, it is necessary to understand charge exchange at the interface and spatial distribution of charge carriers in order to design devices. For this purpose, KFM can be used for exploring the local electrical properties of both single-layer graphene (SLG) and few-layer graphene (FLG) films. Quantitative measurements are taken by detecting the impact of the film thickness on the surface potential. KFM imaging of SLG on silica is shown in Figure 2.

Figure 3. Single-pass KFM and dC/dZ imaging of few-layer graphene on silica. An example of the resulting topography [A] with simultaneously acquired surface potential [B] and capacitance gradient [C] data is displayed. [D] and [E] are the two cursor profiles corresponding to the purple line drawn in topographic image [A] and the blue line in surface potential image [B], respectively.

From Figure 3, it is clear that single-pass KFM and dC/dZ measurements enable the concurrent observation of two unique material properties that demonstrate totally diverse characteristics in response to the graphene layer. This technique can be used to comprehensively characterize and better understand graphene materials.

Applications of Capacitance Gradient dC/dZ Imaging

dC/dZ signals of typical FLG samples that are thicker than five layers are independent of the graphene layers and very distinctive from the substrate. This emphasizes the need to perform a systematic analysis on extensive applications of dC/dZ imaging for characterizing graphene materials. However, one primary question is do AFM-based capacitance gradient measurements have adequate sensitivity to detect SLG.

The ultra-high sensitivity of dC/dZ imaging for SLG detection is demonstrated in Figure 4.

Figure 4. Ultrahigh sensitivity of dC/dZ imaging for detection of single-layer graphene. [A] An optical microscope image showing graphite flakes with various thicknesses on a silica substrate. [B] Raman spectra of the single-layer graphene at a location marked in the optical image. Both AFM topography [C] and corresponding capacitance gradient dC/dZ image [D] of that location.

An exfoliated FLG sample with a few patches of graphene layer on top is illustrated in Figure 5. These tiny patches are not affixed to the underling layers as strongly as the other regions as the AFM tip can disturb them at elevated imaging force. The corresponding capacitance gradient images show that the dC/dZ signals at these sites are darker when compared to the surrounding FLG regions. Thus, the dC/dZ imaging technique holds potential to distinguish the interlayer interactions of the graphene flakes.

Figure 5. Detection of various interlayer interactions in FLG via dC/dZ measurements. [top row] Three selected in situ AFM topographic images showing the disturbance on some loosely bound small patches of graphene flakes by the AFM tip. [bottom row] The corresponding three dC/dZ images.

AFM Analyses of Materials Beyond Graphene

Finding alternative layered inorganic materials that are analogous to graphene but with desired characteristics that graphene lacks is one future direction of graphene-related studies. Monolayer h-BN, also named as ‘white graphene,’ is one such material that shows promise to serve as a complementary substrate for use in graphene electronics. Keysight Technologies recently developed an exclusive technique called scanning microwave microscopy (SMM), which comprises an AFM interfaced with a performance network analyzer (PNA). SMM studies of few-layer h-BN ultrathin films are demonstrated in Figure 6. The results illustrated in Figure 6 exhibits the sensitivity of SMM.

About Keysight Technologies

Keysight is a global electronic measurement technology and market leader helping to transform its customers' measurement experience through innovation in wireless, modular, and software solutions. Keysight provides electronic measurement instruments and systems and related software, software design tools and services used in the design, development, manufacture, installation, deployment and operation of electronic equipment. Information about Keysight is available at www.keysight.com.